A conventional two-wire dimmer has two connections: a “hot” connection to an alternating-current (AC) power supply and a “dimmed hot” connection to the lighting load. Standard dimmers use one or more semiconductor switches, such as triacs or field effect transistors (FETs), to control the current delivered to the lighting load and thus control the intensity of the light. The semiconductor switches are typically coupled between the hot and dimmed hot connections of the dimmer.
Standard wall-mounted dimmers include a user interface having a means for adjusting the intensity of the lighting load, such as a linear slider, a rotary knob, or a rocker switch. Some dimmers also include a button that allows for toggling of the lighting load from off (no power is conducted to the lighting load) to on (power is conducted to the lighting load to illuminate the lighting load). It is desirable to include a status indicator, such as a small lamp or light emitting diode (LED), on the user interface to indicate the status of the lighting load, i.e. on or off.
Many prior art dimmers have incorporated night-lights that are illuminated when the connected lighting load is off and are dimmed or not illuminated when the connected lighting load is on. One example of such a dimmer is disclosed in U.S. Pat. No. 3,864,561, issued Feb. 4, 1975, entitled DIMMER SWITCH WITH ILLUMINATED KNOB. These dimmers include a neon lamp, or an equivalent light source, and a series current-limiting impedance. The neon lamp and the impedance are connected in parallel with the semiconductor switch such that a voltage is developed across the neon lamp when the semiconductor switch is non-conducting. Thus, the neon lamp will have an intensity that is inversely proportional to the intensity of the connected lighting load. However, even though this convention of illuminating the night-light provides an indication of the status of the connected lighting load, the convention is opposite a more common expectation that the status lamp should be on when the connected lighting load is on and off when the load is off.
U.S. Pat. No. 5,017,837, issued on May 21, 1991, entitled INDICATOR LAMP SYSTEM, discloses a load control system that has an indicator lamp that provides a dim light when the load power is off and provides a brighter light when the load power is on. The dim light that is provided when the lighting load is off is bright enough to be seen in a darkened room. However, this system requires many electrical components in addition to the semiconductor switch and related control circuitry for controlling the current through the connected lighting load. This leads to higher costs and limited space on the printed circuit board (PCB) of the dimmer.
Some prior art dimmers are considered “smart” dimmers since they include a microcontroller or other processing means to allow an advanced set of control features and feedback options to the end user. An example of a smart dimmer is disclosed in commonly-assigned U.S. Pat. No. 5,248,919, issued on Sep. 28, 1993, entitled LIGHTING CONTROL DEVICE, which is herein incorporated by reference in its entirety. Smart dimmers typically include a plurality of actuators for receipt of user inputs and a plurality of LEDs for feedback to the user. In order to provide a direct-current (DC) voltage to power the microprocessor and the LEDs, the dimmer includes a power supply. In a two-wire smart dimmer, the power supply can only draw a small current through the lighting load when the semiconductor switch is non-conducting. In order to provide the LED feedback to the user, the smart dimmer is required to include the microprocessor and the power supply, which increase the cost and design requirements of the dimmer.
Thus, there exists a need for a simple status indicator circuit that does not require a power supply or many other components and operates in a manner in which the status indicator is off or at a dim level when the lighting load is off and at a bright level when the load is on.